1. Field of the Invention
The present invention relates to microelectromechanical switches and particularly to capacitive microelectromechanical switches.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [Ref. x]. A list of these different publications ordered according to these reference numbers can be found below at the end of the Detailed Description of the Preferred Embodiment. Each of these publications is incorporated by reference herein.)
Microelectromechanical (MEMS) switches rival the performance of conventional solid-state switching devices, such as positive-intrinsic-negative (PIN) semiconductor diodes and gallium arsenide field effect transistors (GaAs FETs), but at lower costs. In addition, one advantage of using MEMS is their very low additional transmission line loss and distortion.
MEMS switches are micromechanical switches where the active element is a thin metallic membrane movable through the application of a direct current (DC) electrostatic field. These devices have been developed to perform switching of electrical signals with high performance. Currently they are an established technology with great promise for reducing cost and improving performance in certain microwave and millimeter-wave applications.
MEMS switches were first demonstrated in 1979 [Ref. 1] as electrostatically actuated cantilever structures used to perform switching of electrical signals at low frequency. Since then, these switches have demonstrated useful performance at RF and microwave frequencies using cantilever [Ref. 2], rotary [Ref. 3], and membrane [Ref. 4] topologies. Over the past years, the development of radio frequency (RF) MEMS switches with capacitive coupling has been reported by several researchers [Ref. 5-15].
Such capacitive MEMS switches may be implemented using a dielectric layer to effect capacitive coupling when the switch is in the closed position. A crucial factor for the performance of a MEMS switch is the quality of the effective contact of the upper membrane or bridge with the bottom electrode when actuated. It is easy to understand that an imperfect effective contact creates non-predictable electrical behaviors, especially in the value of the achievable closed state capacitance of the switch. Low insertion loss performance can also be seriously deteriorated by a xe2x80x98non-intimatexe2x80x99 contact between the two electrodes. There are several factors that limit the contact. One cause is an air gap created by hillocking in the metal films. Hillocking is a stress relief mechanism that occurs when a metallic thin film is exposed to high temperature. It is often difficult to avoid the formation of roughness on the film and to obtain a perfectly smooth surface [Ref. 16].
There is a need for improved effective contact quality between the bridge and conductor in MEMS switches. There is also a need to create stable and predictable behavior in MEMS switch capacitance to improve performance, particularly in the closed state. There is further a need to minimize the negative effects of hillocking on MEMS switch electrodes. The present invention meets these needs.
The present invention offers the potential for building a new generation of low-loss high-linearity microwave circuits for a variety of radar and communication applications. A microelectromechanical switch is disclosed comprising a conductor, a dielectric layer disposed on the conductor, a metal cap disposed on the dielectric layer and a bridge disposed proximate to the metal cap such that an electrical potential applied between the bridge and conductor causes the bridge to deform and contact the metal cap.
The metal cap presents a stable, controlled surface proximate to the conductor which is used to precisely control the MEMS switch capacitance in the closed or DOWN state. In the DOWN state, the bridge contacts the metal cap and the metal cap principally defines the effective contact between the bridge and conductor. Thus, the present invention creates a greatly improved effective contact between the bridge and conductor through an intervening stable and controllable metal cap.